1. Field of the Invention
The present invention relates to honeycomb panel structures and a method of preparing such structures.
2. Background Information
Diffusion brazing and bonding methods for the joining of honeycomb sandwich panels and other titanium structures are known to those skilled in the art. However, as described for example in U.S. Pat. Nos. 3,768,985 and 3,769,101, such diffusion brazing and bonding techniques have not been found entirely satisfactory, especially for joining titanium-based components such as joining a titanium honeycomb core material to a titanium facing sheet. In response to the problems associated with such diffusion brazing and bonding methods, U.S. Pat. No. 3,768,985 describes a combined brazing and diffusion process referred to as a liquid interface diffusion (LID) process, also known as transient liquid phase (TLP) bonding, for joining a titanium honeycomb core and a titanium facing sheet bonded thereto. A brazing or filler material containing 38% Cu, 38% Ni and a balance of Ag (by weight) is interposed between the faying surfaces of the honeycomb core and facing sheet, and the brazing material is rendered liquid at the brazing temperature to form a liquid interface between the faying surfaces which establishes the required metal-to-metal contact therebetween so that atomic transport can be effected and diffusion accelerated. In addition, U.S. Pat. No. 3,769,101 describes a LID process in which a small amount of three or more selected metals are interposed between the faying edges of the material to be bonded, to form a diffusion bridge. The selected metals may be Cu, Ni and Ag, or Cu, Ni and one low melting point metal selected from the group consisting of Sb, Bi, Cd, Sn, Zn, Au and Pt.
However, there are several additional problems associated with the LID process itself. For example, it is often difficult to achieve sufficient bonding of mismatched faying surfaces. In addition, disbonding of the surfaces after initial bonding also may occur, which may require post-processing repairs such as the introduction of pins and the like to join the surfaces with sufficient mechanical integrity. In view of the foregoing, it would be desirable to employ a process which is capable of sufficiently bonding slightly mismatched faying surfaces, and reduces the incidence of disbanding of the initially bonded surfaces and the concomitant necessity to use pins and the like to sufficiently join the surfaces.
As an alternative to LID, diffusion has been employed as described, for example, in U.S. Pat. No. 4,893,743. However, in order to achieve diffusion bonding, it normally becomes necessary to establish an.ultraclean condition of the parts, and to employ high pressure and temperature for extended periods of time without causing gross deformation and degradation of mechanical properties which might result from use of excessive time, temperature, or pressure. Honeycomb structures typically require a low bonding pressure, to avoid the risk of cracking the core.
The use of amorphous Ti-based brazing alloys for diffusion brazing and bonding of thin sheet structures of titanium and its alloys is described in B. A. Kalin et a., xe2x80x9cBrazing Thin Sheet Structures of Titanium Alloys Using STEMET Amorphous Brazing Alloys,xe2x80x9d in Welding International, pp. 234-35 (1997). However, relatively short bonding times (i.e. 5-30 minutes) are described, which tend to limit the degree of atomic diffusion and homogenization of the joint formed between the honeycomb core and facing sheet.
The use of titanium aluminide (Tixe2x80x94Al) alloys has been described for example in U.S. Pat. No. 5,318,214 and in U.S. Pat. No. 4,869,421 (both patents are herein incorporated by reference in their entireties). As disclosed in the ""214 patent, LID typically requires the system to be maintained for about 80-90 minutes at bonding temperatures. Chaturvedi et al. have disclosed diffusion brazing of an alloy containing titanium, aluminum, niobium, and manganese with a titanium-copper-nickel metal foil, but they have not discussed its application to honeycomb structures.
In view of the above, there is a need in the art for a titanium aluminide honeycomb structure capable of being prepared by brazing with improved braze properties.
Accordingly, it is an object of the present invention to provide titanium aluminide (Tixe2x80x94Al) structures with improved braze properties and a method of preparing such structures. More specifically, it is an object of the present invention to provide a honeycomb structure containing a titanium aluminide alloy honeycomb core joined to at least one Tixe2x80x94Al alloy facing sheet by a diffusion brazed composition zone formed from a metal foil containing copper, titanium, and optionally nickel, which joins the faying surfaces of the honeycomb core and facing sheet(s) by being rendered liquid at the brazing temperature and thereby forming a liquid interface that extensively diffuses into the core and facing sheet(s). It is another object of the invention to provide a method of preparing such a structure. The use of the diffusion brazed composition zone advantageously enables the brazing of such Tixe2x80x94Al alloy components, including the brazing of slightly mismatched faying surfaces of such components, which reduces the incidence of disbonding of the initially joined surfaces and the concomitant necessity to use pins and the like to sufficiently join the surfaces. The structure and method of the invention are useful in applications where high strength, lightweight materials are required, such as in aircraft and aerospace-related applications.
The present invention provides a Tixe2x80x94Al honeycomb panel structure which includes a Tixe2x80x94Al alloy honeycomb core and at least one Tixe2x80x94Al alloy facing sheet brazed thereto with a diffusion brazing composition. The honeycomb structure is prepared by a method including:
(a) providing Tixe2x80x94Al alloy honeycomb core having a faying surface and at least one Tixe2x80x94Al alloy facing sheet having a faying surface;
(b) contacting the honeycomb core faying surface and the at least one facing sheet faying surface, and positioning therebetween a metal braze filler foil containing copper, titanium, and optionally nickel, to form a braze assembly;
(c) subjecting the braze assembly to sufficient positive pressure to maintain position and alignment for joining; and
(d) heating the braze assembly for a sufficient amount of time to join the honeycomb core with the at least one facing sheet.
In a first embodiment, the honeycomb structure is a hybrid Tixe2x80x94Al honeycomb structure in which said the facing sheet(s) is a gamma-based Tixe2x80x94Al (xe2x80x9cxcex3-Tixe2x80x94Alxe2x80x9d) facing sheet having a thickness of 0.007 to 0.040 inches and the core is fabricated from orthorhombic Tixe2x80x94Al (xe2x80x9cOxe2x80x94Tixe2x80x94Alxe2x80x9d) foil gauges having a thickness of about 0.003 inches. In a second embodiment, the honeycomb structure is an all xcex3-Tixe2x80x94Al honeycomb structure, in which the Tixe2x80x94Al alloy used for the fabrication of the core is xcex3-Tixe2x80x94Al and the Tixe2x80x94Al alloy used for the facing sheet(s) is also xcex3-Tixe2x80x94Al. In one particularly preferred embodiment, the core is fabricated from xcex3-Tixe2x80x94Al foil gauges having a thickness of about 0.004 inches. In a third embodiment, the honeycomb structure is an all Oxe2x80x94Tixe2x80x94Al honeycomb structure, in which the Tixe2x80x94Al alloy used for the fabrication of the core is Oxe2x80x94Tixe2x80x94Al and the Tixe2x80x94Al alloy used for the facing sheet(s) is also Oxe2x80x94Tixe2x80x94Al. In one particularly preferred embodiment, the core is fabricated from xcex3-Tixe2x80x94Al foil gauges having a thickness of about 0.003 inches. xcex3-Tixe2x80x94Al alloys may include PM xcex3-MET, supplied by Plansee of Reutte, Austria.
In another preferred embodiment, the metal braze filler foil contains copper, titanium, nickel, and zirconium.
In yet another preferred embodiment, the metal braze filler foil contains copper and titanium and is essentially free of nickel.
In a particularly preferred embodiment, the metal braze filler foil is formed by a rapid solidification process or a melt spinning process.
These and other advantages of the present invention will become more readily apparent from the brief description of the drawings and the detailed description of the invention, which are set forth below.